Sub-block temporal motion vector prediction for video coding

ABSTRACT

A computing device performs a method of decoding video data by determining a co-located picture of the current coding unit; locating a spatial neighbor block of the current coding unit that corresponds to the co-located picture; determining a motion shift vector for the current coding unit from one or more motion vectors associated with the spatial neighbor block according to a predefined fixed order; and reconstructing a sub-block-based temporal motion vector for a respective sub-block of a plurality of sub-blocks in the current coding unit from a corresponding sub-block in the collocated picture based on the motion shift vector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT application No.PCT/US2020/036339, entitled “SUB-BLOCK TEMPORAL MOTION VECTOR PREDICTIONFOR VIDEO CODING” filed on Jun. 5, 2020, which claims the benefit ofU.S. Provisional Application No. 62/858,916, entitled “SUB-BLOCKTEMPORAL MOTION VECTOR PREDICTION FOR VIDEO CODING” filed on Jun. 7,2019, the entire disclosure of both of which is incorporated herein byreference.

TECHNICAL FIELD

The present application generally relates to video data encoding anddecoding, and in particular, to method and system of sub-block motionvector prediction during video data encoding and decoding.

BACKGROUND

Digital video is supported by a variety of electronic devices, such asdigital televisions, laptop or desktop computers, tablet computers,digital cameras, digital recording devices, digital media players, videogaming consoles, smart phones, video teleconferencing devices, videostreaming devices, etc. The electronic devices transmit, receive,encode, decode, and/or store digital video data by implementing videocompression/decompression standards as defined by MPEG-4, ITU-T H.263,ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), HighEfficiency Video Coding (HEVC), and Versatile Video Coding (VVC)standard. Video compression typically includes performing spatial (intraframe) prediction and/or temporal (inter frame) prediction to reduce orremove redundancy inherent in the video data. For block-based videocoding, a video frame is partitioned into one or more slices, each slicehaving multiple video blocks, which may also be referred to as codingtree units (CTUs). Each CTU may contain one coding unit (CU) orrecursively split into smaller CUs until the predefined minimum CU sizeis reached. Each CU (also named leaf CU) contains one or multipletransform units (TUs) and each CU also contains one or multipleprediction units (PUs). Each CU can be coded in either intra, inter orIBC modes. Video blocks in an intra coded (I) slice of a video frame areencoded using spatial prediction with respect to reference samples inneighbor blocks within the same video frame. Video blocks in an intercoded (P or B) slice of a video frame may use spatial prediction withrespect to reference samples in neighbor blocks within the same videoframe or temporal prediction with respect to reference samples in otherprevious and/or future reference video frames.

Spatial or temporal prediction based on a reference block that has beenpreviously encoded, e.g., a neighbor block, results in a predictiveblock for a current video block to be coded. The process of finding thereference block may be accomplished by block matching algorithm.Residual data representing pixel differences between the current blockto be coded and the predictive block is referred to as a residual blockor prediction errors. An inter-coded block is encoded according to amotion vector that points to a reference block in a reference frameforming the predictive block, and the residual block. The process ofdetermining the motion vector is typically referred to as motionestimation. An intra coded block is encoded according to an intraprediction mode and the residual block. For further compression, theresidual block is transformed from the pixel domain to a transformdomain, e.g., frequency domain, resulting in residual transformcoefficients, which may then be quantized. The quantized transformcoefficients, initially arranged in a two-dimensional array, may bescanned to produce a one-dimensional vector of transform coefficients,and then entropy encoded into a video bitstream to achieve even morecompression.

The encoded video bitstream is then saved in a computer-readable storagemedium (e.g., flash memory) to be accessed by another electronic devicewith digital video capability or directly transmitted to the electronicdevice wired or wirelessly. The electronic device then performs videodecompression (which is an opposite process to the video compressiondescribed above) by, e.g., parsing the encoded video bitstream to obtainsyntax elements from the bitstream and reconstructing the digital videodata to its original format from the encoded video bitstream based atleast in part on the syntax elements obtained from the bitstream, andrenders the reconstructed digital video data on a display of theelectronic device.

With digital video quality going from high definition, to 4K×2K or even8K×4K, the amount of vide data to be encoded/decoded growsexponentially. It is a constant challenge in terms of how the video datacan be encoded/decoded more efficiently while maintaining the imagequality of the decoded video data.

SUMMARY

The present application describes implementations related to video dataencoding and decoding and, more particularly, to system and method ofsub-block motion vector prediction.

According to a first aspect of the present application, a method ofdecoding a current coding unit in a current picture, including:determining a co-located picture for the current picture; determining amotion shift vector for the current coding unit according to a motionvector of a spatial neighbor block of the current coding unit, whereinthe motion shift vector indicates a shift in spatial position between arespective sub-block of a plurality of sub-blocks in the current codingunit in the current picture and a corresponding sub-block in theco-located picture; and reconstructing a sub-block-based temporal motionvector for the respective sub-block of the plurality of sub-blocks inthe current coding unit from the corresponding sub-block in theco-located picture based on the motion shift vector.

According to a second aspect of the present application, a computingdevice includes one or more processors, memory and a plurality ofprograms stored in the memory. The programs, when executed by the one ormore processors, cause the computing device to perform operations asdescribed above.

According to a third aspect of the present application, a non-transitorycomputer readable storage medium stores a plurality of programs forexecution by a computing device having one or more processors. Theprograms, when executed by the one or more processors, cause thecomputing device to perform operations as described above.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the implementations and are incorporated herein andconstitute a part of the specification, illustrate the describedimplementations and together with the description serve to explain theunderlying principles. Like reference numerals refer to correspondingparts.

FIG. 1 is a block diagram illustrating an exemplary video encoding anddecoding system in accordance with some implementations of the presentdisclosure.

FIG. 2 is a block diagram illustrating an exemplary video encoder inaccordance with some implementations of the present disclosure.

FIG. 3 is a block diagram illustrating an exemplary video decoder inaccordance with some implementations of the present disclosure.

FIGS. 4A through 4E are block diagrams illustrating how a frame isrecursively partitioned into multiple video blocks of different sizesand shapes in accordance with some implementations of the presentdisclosure.

FIG. 5 is a block diagram illustrating spatially neighboring positionsand temporally co-located block positions of a current CU to be encodedin accordance with some implementations of the present disclosure.

FIGS. 6A-6D are block diagrams illustrating steps for deriving temporalmotion vector predictors of a current block or sub-block temporal motionvector predictors of a sub-block in the current block in accordance withsome implementations of the present disclosure.

FIG. 7 illustrates a block diagram for determining a valid area forderiving a temporal motion vector predictors and sub-block temporalmotion vector predictors in accordance with some implementations of thepresent disclosure.

FIGS. 8A-8B illustrate a flowchart illustrating an exemplary process bywhich a video coder implements the techniques of deriving sub-blocktemporal motion vector predictors in accordance with someimplementations of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to specific implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous non-limiting specific detailsare set forth in order to assist in understanding the subject matterpresented herein. But it will be apparent to one of ordinary skill inthe art that various alternatives may be used without departing from thescope of claims and the subject matter may be practiced without thesespecific details. For example, it will be apparent to one of ordinaryskill in the art that the subject matter presented herein can beimplemented on many types of electronic devices with digital videocapabilities.

FIG. 1 is a block diagram illustrating an exemplary system 10 forencoding and decoding video blocks in parallel in accordance with someimplementations of the present disclosure. As shown in FIG. 1, system 10includes a source device 12 that generates and encodes video data to bedecoded at a later time by a destination device 14. Source device 12 anddestination device 14 may comprise any of a wide variety of electronicdevices, including desktop or laptop computers, tablet computers, smartphones, set-top boxes, digital televisions, cameras, display devices,digital media players, video gaming consoles, video streaming device, orthe like. In some implementations, source device 12 and destinationdevice 14 are equipped with wireless communication capabilities.

In some implementations, destination device 14 may receive the encodedvideo data to be decoded via a link 16. Link 16 may comprise any type ofcommunication medium or device capable of moving the encoded video datafrom source device 12 to destination device 14. In one example, link 16may comprise a communication medium to enable source device 12 totransmit the encoded video data directly to destination device 14 inreal-time. The encoded video data may be modulated according to acommunication standard, such as a wireless communication protocol, andtransmitted to destination device 14. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 12 to destination device 14.

In some other implementations, the encoded video data may be transmittedfrom output interface 22 to a storage device 32. Subsequently, theencoded video data in storage device 32 may be accessed by destinationdevice 14 via input interface 28. Storage device 32 may include any of avariety of distributed or locally accessed data storage media such as ahard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile ornon-volatile memory, or any other suitable digital storage media forstoring encoded video data. In a further example, storage device 32 maycorrespond to a file server or another intermediate storage device thatmay hold the encoded video data generated by source device 12.Destination device 14 may access the stored video data from storagedevice 32 via streaming or downloading. The file server may be any typeof computer capable of storing encoded video data and transmitting theencoded video data to destination device 14. Exemplary file serversinclude a web server (e.g., for a website), an FTP server, networkattached storage (NAS) devices, or a local disk drive. Destinationdevice 14 may access the encoded video data through any standard dataconnection, including a wireless channel (e.g., a Wi-Fi connection), awired connection (e.g., DSL, cable modem, etc.), or a combination ofboth that is suitable for accessing encoded video data stored on a fileserver. The transmission of encoded video data from storage device 32may be a streaming transmission, a download transmission, or acombination of both.

As shown in FIG. 1, source device 12 includes a video source 18, a videoencoder 20 and an output interface 22. Video source 18 may include asource such as a video capture device, e.g., a video camera, a videoarchive containing previously captured video, a video feed interface toreceive video from a video content provider, and/or a computer graphicssystem for generating computer graphics data as the source video, or acombination of such sources. As one example, if video source 18 is avideo camera of a security surveillance system, source device 12 anddestination device 14 may form camera phones or video phones. However,the implementations described in the present application may beapplicable to video coding in general, and may be applied to wirelessand/or wired applications.

The captured, pre-captured, or computer-generated video may be encodedby video encoder 20. The encoded video data may be transmitted directlyto destination device 14 via output interface 22 of source device 12.The encoded video data may also (or alternatively) be stored ontostorage device 32 for later access by destination device 14 or otherdevices, for decoding and/or playback. Output interface 22 may furtherinclude a modem and/or a transmitter.

Destination device 14 includes an input interface 28, a video decoder30, and a display device 34. Input interface 28 may include a receiverand/or a modem and receive the encoded video data over link 16. Theencoded video data communicated over link 16, or provided on storagedevice 32, may include a variety of syntax elements generated by videoencoder 20 for use by video decoder 30 in decoding the video data. Suchsyntax elements may be included within the encoded video datatransmitted on a communication medium, stored on a storage medium, orstored a file server.

In some implementations, destination device 14 may include a displaydevice 34, which can be an integrated display device and an externaldisplay device that is configured to communicate with destination device14. Display device 34 displays the decoded video data to a user, and maycomprise any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according toproprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10,Advanced Video Coding (AVC), or extensions of such standards. It shouldbe understood that the present application is not limited to a specificvideo coding/decoding standard and may be applicable to other videocoding/decoding standards. It is generally contemplated that videoencoder 20 of source device 12 may be configured to encode video dataaccording to any of these current or future standards. Similarly, it isalso generally contemplated that video decoder 30 of destination device14 may be configured to decode video data according to any of thesecurrent or future standards.

Video encoder 20 and video decoder 30 each may be implemented as any ofa variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When implemented partially in software, an electronic devicemay store instructions for the software in a suitable, non-transitorycomputer-readable medium and execute the instructions in hardware usingone or more processors to perform the video coding/decoding operationsdisclosed in the present disclosure. Each of video encoder 20 and videodecoder 30 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device.

FIG. 2 is a block diagram illustrating an exemplary video encoder 20 inaccordance with some implementations described in the presentapplication. Video encoder 20 may perform intra and inter predictivecoding of video blocks within video frames. Intra predictive codingrelies on spatial prediction to reduce or remove spatial redundancy invideo data within a given video frame or picture. Inter predictivecoding relies on temporal prediction to reduce or remove temporalredundancy in video data within adjacent video frames or pictures of avideo sequence.

As shown in FIG. 2, video encoder 20 includes video data memory 40,prediction processing unit 41, decoded picture buffer (DPB) 64, summer50, transform processing unit 52, quantization unit 54, and entropyencoding unit 56. Prediction processing unit 41 further includes motionestimation unit 42, motion compensation unit 44, partition unit 45,intra prediction processing unit 46, and intra block copy (BC) unit 48.In some implementations, video encoder 20 also includes inversequantization unit 58, inverse transform processing unit 60, and summer62 for video block reconstruction. A deblocking filter (not shown) maybe positioned between summer 62 and DPB 64 to filter block boundaries toremove blockiness artifacts from reconstructed video. An in loop filter(not shown) may also be used in addition to the deblocking filter tofilter the output of summer 62. Video encoder 20 may take the form of afixed or programmable hardware unit or may be divided among one or moreof the illustrated fixed or programmable hardware units.

Video data memory 40 may store video data to be encoded by thecomponents of video encoder 20. The video data in video data memory 40may be obtained, for example, from video source 18. DPB 64 is a bufferthat stores reference video data for use in encoding video data by videoencoder 20 (e.g., in intra or inter predictive coding modes). Video datamemory 40 and DPB 64 may be formed by any of a variety of memorydevices. In various examples, video data memory 40 may be on-chip withother components of video encoder 20, or off-chip relative to thosecomponents.

As shown in FIG. 2, after receiving video data, partition unit 45 withinprediction processing unit 41 partitions the video data into videoblocks. This partitioning may also include partitioning a video frameinto slices, tiles, or other larger coding units (CUs) according to apredefined splitting structures such as quad-tree structure associatedwith the video data. The video frame may be divided into multiple videoblocks (or sets of video blocks referred to as tiles). Predictionprocessing unit 41 may select one of a plurality of possible predictivecoding modes, such as one of a plurality of intra predictive codingmodes or one of a plurality of inter predictive coding modes, for thecurrent video block based on error results (e.g., coding rate and thelevel of distortion). Prediction processing unit 41 may provide theresulting intra or inter prediction coded block to summer 50 to generatea residual block and to summer 62 to reconstruct the encoded block foruse as part of a reference frame subsequently. Prediction processingunit 41 also provides syntax elements, such as motion vectors,intra-mode indicators, partition information, and other such syntaxinformation, to entropy encoding unit 56.

In order to select an appropriate intra predictive coding mode for thecurrent video block, intra prediction processing unit 46 withinprediction processing unit 41 may perform intra predictive coding of thecurrent video block relative to one or more neighbor blocks in the sameframe as the current block to be coded to provide spatial prediction.Motion estimation unit 42 and motion compensation unit 44 withinprediction processing unit 41 perform inter predictive coding of thecurrent video block relative to one or more predictive blocks in one ormore reference frames to provide temporal prediction. Video encoder 20may perform multiple coding passes, e.g., to select an appropriatecoding mode for each block of video data.

In some implementations, motion estimation unit 42 determines the interprediction mode for a current video frame by generating a motion vector,which indicates the displacement of a prediction unit (PU) of a videoblock within the current video frame relative to a predictive blockwithin a reference video frame, according to a predetermined patternwithin a sequence of video frames. Motion estimation, performed bymotion estimation unit 42, is the process of generating motion vectors,which estimate motion for video blocks. A motion vector, for example,may indicate the displacement of a PU of a video block within a currentvideo frame or picture relative to a predictive block within a referenceframe (or other coded unit) relative to the current block being codedwithin the current frame (or other coded unit). The predeterminedpattern may designate video frames in the sequence as P frames or Bframes. Intra BC unit 48 may determine vectors, e.g., block vectors, forintra BC coding in a manner similar to the determination of motionvectors by motion estimation unit 42 for inter prediction, or mayutilize motion estimation unit 42 to determine the block vector.

A predictive block is a block of a reference frame that is deemed asclosely matching the PU of the video block to be coded in terms of pixeldifference, which may be determined by sum of absolute difference (SAD),sum of square difference (SSD), or other difference metrics. In someimplementations, video encoder 20 may calculate values for sub-integerpixel positions of reference frames stored in DPB 64. For example, videoencoder 20 may interpolate values of one-quarter pixel positions,one-eighth pixel positions, or other fractional pixel positions of thereference frame. Therefore, motion estimation unit 42 may perform amotion search relative to the full pixel positions and fractional pixelpositions and output a motion vector with fractional pixel precision.

Motion estimation unit 42 calculates a motion vector for a PU of a videoblock in an inter prediction coded frame by comparing the position ofthe PU to the position of a predictive block of a reference frameselected from a first reference frame list (List 0) or a secondreference frame list (List 1), each of which identifies one or morereference frames stored in DPB 64. Motion estimation unit 42 sends thecalculated motion vector to motion compensation unit 44 and then toentropy encoding unit 56.

Motion compensation, performed by motion compensation unit 44, mayinvolve fetching or generating the predictive block based on the motionvector determined by motion estimation unit 42. Upon receiving themotion vector for the PU of the current video block, motion compensationunit 44 may locate a predictive block to which the motion vector pointsin one of the reference frame lists, retrieve the predictive block fromDPB 64, and forward the predictive block to summer 50. Summer 50 thenforms a residual video block of pixel difference values by subtractingpixel values of the predictive block provided by motion compensationunit 44 from the pixel values of the current video block being coded.The pixel difference values forming the residual vide block may includeluma or chroma difference components or both. Motion compensation unit44 may also generate syntax elements associated with the video blocks ofa video frame for use by video decoder 30 in decoding the video blocksof the video frame. The syntax elements may include, for example, syntaxelements defining the motion vector used to identify the predictiveblock, any flags indicating the prediction mode, or any other syntaxinformation described herein. Note that motion estimation unit 42 andmotion compensation unit 44 may be highly integrated, but areillustrated separately for conceptual purposes.

In some implementations, intra BC unit 48 may generate vectors and fetchpredictive blocks in a manner similar to that described above inconnection with motion estimation unit 42 and motion compensation unit44, but with the predictive blocks being in the same frame as thecurrent block being coded and with the vectors being referred to asblock vectors as opposed to motion vectors. In particular, intra BC unit48 may determine an intra-prediction mode to use to encode a currentblock. In some examples, intra BC unit 48 may encode a current blockusing various intra-prediction modes, e.g., during separate encodingpasses, and test their performance through rate-distortion analysis.Next, intra BC unit 48 may select, among the various testedintra-prediction modes, an appropriate intra-prediction mode to use andgenerate an intra-mode indicator accordingly. For example, intra BC unit48 may calculate rate-distortion values using a rate-distortion analysisfor the various tested intra-prediction modes, and select theintra-prediction mode having the best rate-distortion characteristicsamong the tested modes as the appropriate intra-prediction mode to use.Rate-distortion analysis generally determines an amount of distortion(or error) between an encoded block and an original, unencoded blockthat was encoded to produce the encoded block, as well as a bitrate(i.e., a number of bits) used to produce the encoded block. Intra BCunit 48 may calculate ratios from the distortions and rates for thevarious encoded blocks to determine which intra-prediction mode exhibitsthe best rate-distortion value for the block.

In other examples, intra BC unit 48 may use motion estimation unit 42and motion compensation unit 44, in whole or in part, to perform suchfunctions for Intra BC prediction according to the implementationsdescribed herein. In either case, for Intra block copy, a predictiveblock may be a block that is deemed as closely matching the block to becoded, in terms of pixel difference, which may be determined by sum ofabsolute difference (SAD), sum of squared difference (SSD), or otherdifference metrics, and identification of the predictive block mayinclude calculation of values for sub-integer pixel positions.

Whether the predictive block is from the same frame according to intraprediction, or a different frame according to inter prediction, videoencoder 20 may form a residual video block by subtracting pixel valuesof the predictive block from the pixel values of the current video blockbeing coded, forming pixel difference values. The pixel differencevalues forming the residual video block may include both luma and chromacomponent differences.

Intra prediction processing unit 46 may intra-predict a current videoblock, as an alternative to the inter-prediction performed by motionestimation unit 42 and motion compensation unit 44, or the intra blockcopy prediction performed by intra BC unit 48, as described above. Inparticular, intra prediction processing unit 46 may determine an intraprediction mode to use to encode a current block. To do so, intraprediction processing unit 46 may encode a current block using variousintra prediction modes, e.g., during separate encoding passes, and intraprediction processing unit 46 (or a mode select unit, in some examples)may select an appropriate intra prediction mode to use from the testedintra prediction modes. Intra prediction processing unit 46 may provideinformation indicative of the selected intra-prediction mode for theblock to entropy encoding unit 56. Entropy encoding unit 56 may encodethe information indicating the selected intra-prediction mode in thebitstream.

After prediction processing unit 41 determines the predictive block forthe current video block via either inter prediction or intra prediction,summer 50 forms a residual video block by subtracting the predictiveblock from the current video block. The residual video data in theresidual block may be included in one or more transform units (TUs) andis provided to transform processing unit 52. Transform processing unit52 transforms the residual video data into residual transformcoefficients using a transform, such as a discrete cosine transform(DCT) or a conceptually similar transform.

Transform processing unit 52 may send the resulting transformcoefficients to quantization unit 54. Quantization unit 54 quantizes thetransform coefficients to further reduce bit rate. The quantizationprocess may also reduce the bit depth associated with some or all of thecoefficients. The degree of quantization may be modified by adjusting aquantization parameter. In some examples, quantization unit 54 may thenperform a scan of a matrix including the quantized transformcoefficients. Alternatively, entropy encoding unit 56 may perform thescan.

Following quantization, entropy encoding unit 56 entropy encodes thequantized transform coefficients into a video bitstream using, e.g.,context adaptive variable length coding (CAVLC), context adaptive binaryarithmetic coding (CABAC), syntax-based context-adaptive binaryarithmetic coding (SBAC), probability interval partitioning entropy(PIPE) coding or another entropy encoding methodology or technique. Theencoded bitstream may then be transmitted to video decoder 30, orarchived in storage device 32 for later transmission to or retrieval byvideo decoder 30. Entropy encoding unit 56 may also entropy encode themotion vectors and the other syntax elements for the current video framebeing coded.

Inverse quantization unit 58 and inverse transform processing unit 60apply inverse quantization and inverse transformation, respectively, toreconstruct the residual video block in the pixel domain for generatinga reference block for prediction of other video blocks. As noted above,motion compensation unit 44 may generate a motion compensated predictiveblock from one or more reference blocks of the frames stored in DPB 64.Motion compensation unit 44 may also apply one or more interpolationfilters to the predictive block to calculate sub-integer pixel valuesfor use in motion estimation.

Summer 62 adds the reconstructed residual block to the motioncompensated predictive block produced by motion compensation unit 44 toproduce a reference block for storage in DPB 64. The reference block maythen be used by intra BC unit 48, motion estimation unit 42 and motioncompensation unit 44 as a predictive block to inter predict anothervideo block in a subsequent video frame.

FIG. 3 is a block diagram illustrating an exemplary video decoder 30 inaccordance with some implementations of the present application. Videodecoder 30 includes video data memory 79, entropy decoding unit 80,prediction processing unit 81, inverse quantization unit 86, inversetransform processing unit 88, summer 90, and DPB 92. Predictionprocessing unit 81 further includes motion compensation unit 82, intraprediction unit 84, and intra BC unit 85. Video decoder 30 may perform adecoding process generally reciprocal to the encoding process describedabove with respect to video encoder 20 in connection with FIG. 2. Forexample, motion compensation unit 82 may generate prediction data basedon motion vectors received from entropy decoding unit 80, whileintra-prediction unit 84 may generate prediction data based onintra-prediction mode indicators received from entropy decoding unit 80.

In some examples, a unit of video decoder 30 may be tasked to performthe implementations of the present application. Also, in some examples,the implementations of the present disclosure may be divided among oneor more of the units of video decoder 30. For example, intra BC unit 85may perform the implementations of the present application, alone, or incombination with other units of video decoder 30, such as motioncompensation unit 82, intra prediction unit 84, and entropy decodingunit 80. In some examples, video decoder 30 may not include intra BCunit 85 and the functionality of intra BC unit 85 may be performed byother components of prediction processing unit 81, such as motioncompensation unit 82.

Video data memory 79 may store video data, such as an encoded videobitstream, to be decoded by the other components of video decoder 30.The video data stored in video data memory 79 may be obtained, forexample, from storage device 32, from a local video source, such as acamera, via wired or wireless network communication of video data, or byaccessing physical data storage media (e.g., a flash drive or harddisk). Video data memory 79 may include a coded picture buffer (CPB)that stores encoded video data from an encoded video bitstream. Decodedpicture buffer (DPB) 92 of video decoder 30 stores reference video datafor use in decoding video data by video decoder 30 (e.g., in intra orinter predictive coding modes). Video data memory 79 and DPB 92 may beformed by any of a variety of memory devices, such as dynamic randomaccess memory (DRAM), including synchronous DRAM (SDRAM),magneto-resistive RAM (MRAM), resistive RAM (RRAM), or other types ofmemory devices. For illustrative purpose, video data memory 79 and DPB92 are depicted as two distinct components of video decoder 30 in FIG.3. But it will be apparent to one skilled in the art that video datamemory 79 and DPB 92 may be provided by the same memory device orseparate memory devices. In some examples, video data memory 79 may beon-chip with other components of video decoder 30, or off-chip relativeto those components.

During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video frame andassociated syntax elements. Video decoder 30 may receive the syntaxelements at the video frame level and/or the video block level. Entropydecoding unit 80 of video decoder 30 entropy decodes the bitstream togenerate quantized coefficients, motion vectors or intra-prediction modeindicators, and other syntax elements. Entropy decoding unit 80 thenforwards the motion vectors and other syntax elements to predictionprocessing unit 81.

When the video frame is coded as an intra predictive coded (I) frame orfor intra coded predictive blocks in other types of frames, intraprediction unit 84 of prediction processing unit 81 may generateprediction data for a video block of the current video frame based on asignaled intra prediction mode and reference data from previouslydecoded blocks of the current frame.

When the video frame is coded as an inter-predictive coded (i.e., B orP) frame, motion compensation unit 82 of prediction processing unit 81produces one or more predictive blocks for a video block of the currentvideo frame based on the motion vectors and other syntax elementsreceived from entropy decoding unit 80. Each of the predictive blocksmay be produced from a reference frame within one of the reference framelists. Video decoder 30 may construct the reference frame lists, List 0and List 1, using default construction techniques based on referenceframes stored in DPB 92.

In some examples, when the video block is coded according to the intraBC mode described herein, intra BC unit 85 of prediction processing unit81 produces predictive blocks for the current video block based on blockvectors and other syntax elements received from entropy decoding unit80. The predictive blocks may be within a reconstructed region of thesame picture as the current video block defined by video encoder 20.

Motion compensation unit 82 and/or intra BC unit 85 determinesprediction information for a video block of the current video frame byparsing the motion vectors and other syntax elements, and then uses theprediction information to produce the predictive blocks for the currentvideo block being decoded. For example, motion compensation unit 82 usessome of the received syntax elements to determine a prediction mode(e.g., intra or inter prediction) used to code video blocks of the videoframe, an inter prediction frame type (e.g., B or P), constructioninformation for one or more of the reference frame lists for the frame,motion vectors for each inter predictive encoded video block of theframe, inter prediction status for each inter predictive coded videoblock of the frame, and other information to decode the video blocks inthe current video frame.

Similarly, intra BC unit 85 may use some of the received syntaxelements, e.g., a flag, to determine that the current video block waspredicted using the intra BC mode, construction information of whichvideo blocks of the frame are within the reconstructed region and shouldbe stored in DPB 92, block vectors for each intra BC predicted videoblock of the frame, intra BC prediction status for each intra BCpredicted video block of the frame, and other information to decode thevideo blocks in the current video frame.

Motion compensation unit 82 may also perform interpolation using theinterpolation filters as used by video encoder 20 during encoding of thevideo blocks to calculate interpolated values for sub-integer pixels ofreference blocks. In this case, motion compensation unit 82 maydetermine the interpolation filters used by video encoder 20 from thereceived syntax elements and use the interpolation filters to producepredictive blocks.

Inverse quantization unit 86 inverse quantizes the quantized transformcoefficients provided in the bitstream and entropy decoded by entropydecoding unit 80 using the same quantization parameter calculated byvideo encoder 20 for each video block in the video frame to determine adegree of quantization. Inverse transform processing unit 88 applies aninverse transform, e.g., an inverse DCT, an inverse integer transform,or a conceptually similar inverse transform process, to the transformcoefficients in order to reconstruct the residual blocks in the pixeldomain.

After motion compensation unit 82 or intra BC unit 85 generates thepredictive block for the current video block based on the vectors andother syntax elements, summer 90 reconstructs decoded video block forthe current video block by summing the residual block from inversetransform processing unit 88 and a corresponding predictive blockgenerated by motion compensation unit 82 and intra BC unit 85. Anin-loop filter (not pictured) may be positioned between summer 90 andDPB 92 to further process the decoded video block. The decoded videoblocks in a given frame are then stored in DPB 92, which storesreference frames used for subsequent motion compensation of next videoblocks. DPB 92, or a memory device separate from DPB 92, may also storedecoded video for later presentation on a display device, such asdisplay device 34 of FIG. 1.

In a typical video coding process, a video sequence typically includesan ordered set of frames or pictures. Each frame may include threesample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional arrayof luma samples. SCb is a two-dimensional array of Cb chroma samples.SCr is a two-dimensional array of Cr chroma samples. In other instances,a frame may be monochrome and therefore includes only onetwo-dimensional array of luma samples.

As shown in FIG. 4A, video encoder 20 (or more specifically partitionunit 45) generates an encoded representation of a frame by firstpartitioning the frame into a set of coding tree units (CTUs). A videoframe may include an integer number of CTUs ordered consecutively in araster scan order from left to right and from top to bottom. Each CTU isa largest logical coding unit and the width and height of the CTU aresignaled by the video encoder 20 in a sequence parameter set, such thatall the CTUs in a video sequence have the same size being one of128×128, 64×64, 32×32, and 16×16. But it should be noted that thepresent application is not necessarily limited to a particular size. Asshown in FIG. 4B, each CTU may comprise one coding tree block (CTB) ofluma samples, two corresponding coding tree blocks of chroma samples,and syntax elements used to code the samples of the coding tree blocks.The syntax elements describe properties of different types of units of acoded block of pixels and how the video sequence can be reconstructed atthe video decoder 30, including inter or intra prediction, intraprediction mode, motion vectors, and other parameters. In monochromepictures or pictures having three separate color planes, a CTU maycomprise a single coding tree block and syntax elements used to code thesamples of the coding tree block. A coding tree block may be an N×Nblock of samples.

To achieve a better performance, video encoder 20 may recursivelyperform tree partitioning such as binary-tree partitioning, ternary-treepartitioning, quad-tree partitioning or a combination of both on thecoding tree blocks of the CTU and divide the CTU into smaller codingunits (CUs). As depicted in FIG. 4C, the 64×64 CTU 400 is first dividedinto four smaller CU, each having a block size of 32×32. Among the foursmaller CUs, CU 410 and CU 420 are each divided into four CUs of 16×16by block size. The two 16×16 CUs 430 and 440 are each further dividedinto four CUs of 8×8 by block size. FIG. 4D depicts a quad-tree datastructure illustrating the end result of the partition process of theCTU 400 as depicted in FIG. 4C, each leaf node of the quad-treecorresponding to one CU of a respective size ranging from 32×32 to 8×8.Like the CTU depicted in FIG. 4B, each CU may comprise a coding block(CB) of luma samples and two corresponding coding blocks of chromasamples of a frame of the same size, and syntax elements used to codethe samples of the coding blocks. In monochrome pictures or pictureshaving three separate color planes, a CU may comprise a single codingblock and syntax structures used to code the samples of the codingblock. It should be noted that the quad-tree partitioning depicted inFIGS. 4C and 4D is only for illustrative purposes and one CTU can besplit into CUs to adapt to varying local characteristics based onquad/ternary/binary-tree partitions. In the multi-type tree structure,one CTU is partitioned by a quad-tree structure and each quad-tree leafCU can be further partitioned by a binary and ternary tree structure. Asshown in FIG. 4E, there are five possible partitioning types of a codingblock having a width W and a height H, i.e., quaternary partitioning,horizontal binary partitioning, vertical binary partitioning, horizontalternary partitioning, and vertical ternary partitioning.

In some implementations, video encoder 20 may further partition a codingblock of a CU into one or more M×N prediction blocks (PB). A predictionblock is a rectangular (square or non-square) block of samples on whichthe same prediction, inter or intra, is applied. A prediction unit (PU)of a CU may comprise a prediction block of luma samples, twocorresponding prediction blocks of chroma samples, and syntax elementsused to predict the prediction blocks. In monochrome pictures orpictures having three separate color planes, a PU may comprise a singleprediction block and syntax structures used to predict the predictionblock. Video encoder 20 may generate predictive luma, Cb, and Cr blocksfor luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder 20 may use intra prediction or inter prediction togenerate the predictive blocks for a PU. If video encoder 20 uses intraprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofthe frame associated with the PU. If video encoder 20 uses interprediction to generate the predictive blocks of a PU, video encoder 20may generate the predictive blocks of the PU based on decoded samples ofone or more frames other than the frame associated with the PU.

After video encoder 20 generates predictive luma, Cb, and Cr blocks forone or more PUs of a CU, video encoder 20 may generate a luma residualblock for the CU by subtracting the CU's predictive luma blocks from itsoriginal luma coding block such that each sample in the CU's lumaresidual block indicates a difference between a luma sample in one ofthe CU's predictive luma blocks and a corresponding sample in the CU'soriginal luma coding block. Similarly, video encoder 20 may generate aCb residual block and a Cr residual block for the CU, respectively, suchthat each sample in the CU's Cb residual block indicates a differencebetween a Cb sample in one of the CU's predictive Cb blocks and acorresponding sample in the CU's original Cb coding block and eachsample in the CU's Cr residual block may indicate a difference between aCr sample in one of the CU's predictive Cr blocks and a correspondingsample in the CU's original Cr coding block.

Furthermore, as illustrated in FIG. 4C, video encoder 20 may usequad-tree partitioning to decompose the luma, Cb, and Cr residual blocksof a CU into one or more luma, Cb, and Cr transform blocks. A transformblock is a rectangular (square or non-square) block of samples on whichthe same transform is applied. A transform unit (TU) of a CU maycomprise a transform block of luma samples, two corresponding transformblocks of chroma samples, and syntax elements used to transform thetransform block samples. Thus, each TU of a CU may be associated with aluma transform block, a Cb transform block, and a Cr transform block. Insome examples, the luma transform block associated with the TU may be asub-block of the CU's luma residual block. The Cb transform block may bea sub-block of the CU's Cb residual block. The Cr transform block may bea sub-block of the CU's Cr residual block. In monochrome pictures orpictures having three separate color planes, a TU may comprise a singletransform block and syntax structures used to transform the samples ofthe transform block.

Video encoder 20 may apply one or more transforms to a luma transformblock of a TU to generate a luma coefficient block for the TU. Acoefficient block may be a two-dimensional array of transformcoefficients. A transform coefficient may be a scalar quantity. Videoencoder 20 may apply one or more transforms to a Cb transform block of aTU to generate a Cb coefficient block for the TU. Video encoder 20 mayapply one or more transforms to a Cr transform block of a TU to generatea Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, aCb coefficient block or a Cr coefficient block), video encoder 20 mayquantize the coefficient block. Quantization generally refers to aprocess in which transform coefficients are quantized to possibly reducethe amount of data used to represent the transform coefficients,providing further compression. After video encoder 20 quantizes acoefficient block, video encoder 20 may entropy encode syntax elementsindicating the quantized transform coefficients. For example, videoencoder 20 may perform Context-Adaptive Binary Arithmetic Coding (CABAC)on the syntax elements indicating the quantized transform coefficients.Finally, video encoder 20 may output a bitstream that includes asequence of bits that forms a representation of coded frames andassociated data, which is either saved in storage device 32 ortransmitted to destination device 14.

After receiving a bitstream generated by video encoder 20, video decoder30 may parse the bitstream to obtain syntax elements from the bitstream.Video decoder 30 may reconstruct the frames of the video data based atleast in part on the syntax elements obtained from the bitstream. Theprocess of reconstructing the video data is generally reciprocal to theencoding process performed by video encoder 20. For example, videodecoder 30 may perform inverse transforms on the coefficient blocksassociated with TUs of a current CU to reconstruct residual blocksassociated with the TUs of the current CU. Video decoder 30 alsoreconstructs the coding blocks of the current CU by adding the samplesof the predictive blocks for PUs of the current CU to correspondingsamples of the transform blocks of the TUs of the current CU. Afterreconstructing the coding blocks for each CU of a frame, video decoder30 may reconstruct the frame.

As noted above, video coding achieves video compression using primarilytwo modes, i.e., intra-frame prediction (or intra-prediction) andinter-frame prediction (or inter-prediction). It is noted that IBC couldbe regarded as either intra-frame prediction or a third mode. Betweenthe two modes, inter-frame prediction contributes more to the codingefficiency than intra-frame prediction because of the use of motionvectors for predicting a current video block from a reference videoblock.

But with the ever improving video data capturing technology and morerefined video block size for preserving details in the video data, theamount of data required for representing motion vectors for a currentframe also increases substantially. One way of overcoming this challengeis to benefit from the fact that not only a group of neighboring CUs inboth the spatial and temporal domains have similar video data forpredicting purpose but the motion vectors between these neighboring CUsare also similar. Therefore, it is possible to use the motioninformation of spatially neighboring CUs and/or temporally co-locatedCUs as an approximation of the motion information (e.g., motion vector)of a current CU by exploring their spatial and temporal correlation,which is also referred to as “motion vector predictor” (MVP) of thecurrent CU.

Instead of encoding, into the video bitstream, an actual motion vectorof the current CU determined by motion estimation unit 42 as describedabove in connection with FIG. 2, the motion vector predictor of thecurrent CU is subtracted from the actual motion vector of the current CUto produce a motion vector difference (MVD) for the current CU. By doingso, there is no need to encode the motion vector determined by motionestimation unit 42 for each CU of a frame into the video bitstream andthe amount of data used for representing motion information in the videobitstream can be significantly decreased.

Like the process of choosing a predictive block in a reference frameduring inter-frame prediction of a code block, a set of rules need to beadopted by both video encoder 20 and video decoder 30 for constructing amotion vector candidate list (also known as a “merge list”) for acurrent CU using those potential candidate motion vectors associatedwith spatially neighboring CUs and/or temporally co-located CUs of thecurrent CU and then selecting one member from the motion vectorcandidate list as a motion vector predictor for the current CU. By doingso, there is no need to transmit the motion vector candidate list itselfbetween video encoder 20 and video decoder 30 and an index of theselected motion vector predictor within the motion vector candidate listis sufficient for video encoder 20 and video decoder 30 to use the samemotion vector predictor within the motion vector candidate list forencoding and decoding the current CU.

In some implementations, each inter-prediction CU has three motionvector prediction modes including inter (which is also referred to as“advanced motion vector prediction” (AMVP)), skip, and merge forconstructing the motion vector candidate list. Under each mode, one ormore motion vector candidates may be added to the motion vectorcandidate list according to the algorithms described below. Ultimatelyone of them in the candidate list is used as the best motion vectorpredictor of the inter-prediction CU to be encoded into the videobitstream by video encoder 20 or decoded from the video bitstream byvideo decoder 30. To find the best motion vector predictor from thecandidate list, a motion vector competition (MVC) scheme is introducedto select a motion vector from a given candidate set of motion vectors,i.e., the motion vector candidate list, that includes spatial andtemporal motion vector candidates.

In addition to deriving motion vector predictor candidates fromspatially neighboring or temporally co-located CUs, the motion vectorpredictor candidates can also be derived from the so-called“history-based motion vector prediction” (HMVP) table. The HMVP tablehouses a predefined number of motion vector predictors, each having beenused for encoding/decoding a particular CU of the same row of CTUs (orsometimes the same CTU). Because of the spatial/temporal proximity ofthese CUs, there is a high likelihood that one of the motion vectorpredictors in the HMVP table may be reused for encoding/decodingdifferent CUs within the same row of CTUs. Therefore, it is possible toachieve a higher code efficiency by including the HMVP table in theprocess of constructing the motion vector candidate list.

In some implementations, the HMVP table has a fixed length (e.g., 5) andis managed in a quasi-First-In-First-Out (FIFO) manner. For example, amotion vector is reconstructed for a CU when decoding one inter-codedblock of the CU. The HMVP table is updated on-the-fly with thereconstructed motion vector because such motion vector could be themotion vector predictor of a subsequent CU. When updating the HMVPtable, there are two scenarios: (i) the reconstructed motion vector isdifferent from other existing motion vectors in the HMVP table or (ii)the reconstructed motion vector is the same as one of the existingmotion vectors in the HMVP table. For the first scenario, thereconstructed motion vector is added to the HMVP table as the newest oneif the HMVP table is not full. If the HMVP table is already full, theoldest motion vector in the HMVP table needs to be removed from the HMVPtable first before the reconstructed motion vector is added as thenewest one. In other words, the HMVP table in this case is similar to aFIFO buffer such that the motion information located at the head of theFIFO buffer and associated with another previously inter-coded block isshifted out of the buffer so that the reconstructed motion vector isappended to the tail of the FIFO buffer as the newest member in the HMVPtable. For the second scenario, the existing motion vector in the HMVPtable that is substantially identical to the reconstructed motion vectoris removed from the HMVP table before the reconstructed motion vector isadded to the HMVP table as the newest one. If the HMVP table is alsomaintained in the form of a FIFO buffer, the motion vector predictorsafter the identical motion vector in the HMVP table are shifted forwardby one element to occupy the space left by the removed motion vector andthe reconstructed motion vector is then appended to the tail of the FIFObuffer as the newest member in the HMVP table.

The motion vectors in the HMVP table could be added to the motion vectorcandidate lists under different prediction modes such as AMVP, merge,skip, etc. It has been found that the motion information of previouslyinter-coded blocks stored in the HMVP table even not adjacent to thecurrent block can be utilized for more efficient motion vectorprediction.

After one MVP candidate is selected within the given candidate set ofmotion vectors for a current CU, video encoder 20 may generate one ormore syntax elements for the corresponding MVP candidate and encode theminto the video bitstream such that video decoder 30 can retrieve the MVPcandidate from the video bitstream using the syntax elements. Dependingon the specific mode used for constructing the motion vectors candidateset, different modes (e.g., AMVP, merge, skip, etc.) have different setsof syntax elements. For the AMVP mode, the syntax elements include interprediction indicators (List 0, List 1, or bi-directional prediction),reference indices, motion vector candidate indices, motion vectorprediction residual signal, etc. For the skip mode and the merge mode,only merge indices are encoded into the bitstream because the current CUinherits the other syntax elements including the inter predictionindicators, reference indices, and motion vectors from a neighboring CUreferred by the coded merge index. In the case of a skip coded CU, themotion vector prediction residual signal is also omitted.

FIG. 5 is a block diagram illustrating spatially neighboring andtemporally co-located block positions of a current CU to beencoded/decoded in accordance with some implementations of the presentdisclosure. For a given mode, a motion vector prediction (MVP) candidatelist is constructed by first checking the availability of motion vectorsassociated with the spatially left and above neighboring blockpositions, and the availability of motion vectors associated withtemporally co-located block positions and then the motion vectors in theHMVP table. During the process of constructing the MVP candidate list,some redundant MVP candidates are removed from the candidate list and,if necessary, zero-valued motion vector is added to make the candidatelist to have a fixed length (note that different modes may havedifferent fixed lengths). After the construction of the MVP candidatelist, video encoder 20 can select the best motion vector predictor fromthe candidate list and encode the corresponding index indicating thechosen candidate into the video bitstream.

In some embodiments, the candidate list (also known as merge candidatelist) is constructed by including the following five types of candidatesin the order of:

-   -   1. Spatial MVP (i.e. motion vector predictor) from spatially        neighboring CUs    -   2. Temporal MVP from co-located CUs    -   3. History-based MVP from a FIFO table    -   4. Pairwise average MVP    -   5. Zero MVs

In some embodiments, the size of the candidate list is signaled in sliceheader and the maximum allowed size of the candidate list is six (e.g.,in VVC). For each CU code in merge mode, an index of best mergecandidate is encoded using truncated unary binarization (TU). The firstbin of the merge index is coded with context and bypass coding is usedfor other bins. In the following context of this disclosure, thisextended merge mode is also called regular merge mode since its conceptis the same as the merge mode used in HEVC.

Using FIG. 5 as an example and assuming that the candidate list has afixed length of two, the motion vector predictor (MVP) candidate listfor the current CU may be constructed by performing the following stepsin order under the AMVP mode:

-   -   1) Selection of MVP candidates from spatially neighboring CUs        -   a) Derive up to one non-scaled MVP candidate from one of the            two left spatial neighbor CUs starting with A0 and ending            with A1;        -   b) If no non-scaled MVP candidate from left is available in            the previous step, derive up to one scaled MVP candidate            from one of the two left spatial neighbor CUs starting with            A0 and ending with A1;        -   c) Derive up to one non-scaled MVP candidate from one of the            three above spatial neighbor CUs starting with B0, then B1,            and ending with B2;        -   d) If neither A0 nor A1 is available or if they are coded in            intra modes, derive up to one scaled MVP candidate from one            of the three above spatial neighbor CUs starting with B0,            then B1, and ending with B2;    -   2) If two MVP candidates are found in the previous steps and        they are identical, remove one of the two candidates from the        MVP candidate list;    -   3) Selection of MVP candidates from temporally co-located CUs        -   a) If the MVP candidate list after the previous step does            not include two MVP candidates, derive up to one MVP            candidate from the temporal co-located CUs (e.g., T0)    -   4) Selection of MVP candidates from the HMVP table        -   a) If the MVP candidate list after the previous step does            not include two MVP candidates, derive up to two            history-based MVP from the HMVP table; and    -   5) If the MVP candidate list after the previous step does not        include two MVP candidates, add up to two zero-valued MVPs to        the MVP candidate list.

Since there are only two candidates in the AMVP-mode MVP candidate listconstructed above, an associated syntax element like a binary flag isencoded into the bitstream to indicate that which of the two MVPcandidates within the candidate list is used for decoding the currentCU.

In some implementations, the MVP candidate list for the current CU underthe skip or merge mode may be constructed by performing a similar set ofsteps in order like the ones above. It is noted that one special kind ofmerge candidate called “pair-wise merge candidate” is also included intothe MVP candidate list for the skip or merge mode. The pair-wise mergecandidate is generated by averaging the MVs of the two previouslyderived merge-mode motion vector candidates. The size of the merge MVPcandidate list (e.g., from 1 to 6) is signaled in a slice header of thecurrent CU. For each CU in the merge mode, an index of the best mergecandidate is encoded using truncated unary binarization (TU). The firstbin of the merge index is coded with context and bypass coding is usedfor other bins.

As mentioned above, the history-based MVPs can be added to either theAMVP-mode MVP candidate list or the merge MVP candidate list after thespatial MVP and temporal MVP. The motion information of a previouslyinter-coded CU is stored in the HMVP table and used as an MVP candidatefor the current CU. The HMVP table is maintained during theencoding/decoding process. Whenever there is a non-sub-block inter-codedCU, the associated motion vector information is added to the last entryof the HMVP table as a new candidate while the motion vector informationstored in the first entry of the HMVP table is removed from therein (ifthe HMVP table is already full and there is no identical duplicate ofthe associated motion vector information in the table). Alternatively,the identical duplicate of the associated motion vector information isremoved from the table before the associated motion vector informationis added to the last entry of the HMVP table.

As noted above, intra block copy (IBC) can significantly improve thecoding efficiency of screen content materials. Since IBC mode isimplemented as a block-level coding mode, block matching (BM) isperformed at video encoder 20 to find an optimal block vector for eachCU. Here, a block vector is used to indicate the displacement from thecurrent block to a reference block, which has already been reconstructedwithin the current picture. An IBC mode is treated as the thirdprediction mode other than the intra or inter prediction modes.

At the CU level, the IBC mode can be signaled as IBC AMVP mode or IBCskip/merge mode as follows:

-   -   IBC AMVP mode: a block vector difference (BVD) between the        actual block vector of a CU and a block vector predictor of the        CU selected from block vector candidates of the CU is encoded in        the same way as a motion vector difference is encoded under the        AMVP mode described above. The block vector prediction method        uses two block vector candidates as predictors, one from left        neighbor and the other one from above neighbor (if IBC coded).        When either neighbor is not available, a default block vector        will be used as a block vector predictor. A binary flag is        signaled to indicate the block vector predictor index. The IBC        AMVP candidate list consists of spatial and HMVP candidates.    -   IBC skip/merge mode: a merge candidate index is used to indicate        which of the block vector candidates in the merge candidate list        (also known as a “merge list” or “candidate list”) from        neighboring IBC coded blocks is used to predict the block vector        for the current block. The IBC merge candidate list consists of        spatial, HMVP, and pairwise candidates.

FIGS. 6A-6D are block diagrams illustrating steps for deriving temporalmotion vector predictors (TMVPs) of a current block or sub-blocktemporal motion vector predictors (SbTMVPS) of a sub-block in accordancewith some implementations of the present disclosure.

In some embodiments, only one temporal motion vector predictor (TMVP)candidate is added to the merge candidate list as described with respectto FIG. 5. A first flag (sps_temporal_mvp_enabled_flag) is signaled inthe sequence parameter set (SPS) of the picture and a second flag(slice_temporal_mvp_enabled_flag) is signaled in the slice header toindicate whether this TMVP candidate is enabled or disabled.Particularly, in the derivation of this temporal merge candidate, ascaled motion vector is derived from MVs of the co-located picture,which is a previously coded picture in a reference picture list. In thederivation of the temporal motion candidate, an explicit flag in sliceheader (co-located_from_l0_flag) is firstly sent to the decoder toindicate whether the co-located picture is selected from the firstreference frame list (List 0) or the second reference frame list (List1). A co-located reference index (co-located_ref_idx) is further sent toindicate which picture in the used list is selected as the co-locatedpicture for deriving the temporal motion candidate. The List 0 (alsoknown as L0) and List 1 (also known as L1) MVs of the temporal motioncandidate is derived independently according to a predefined order forthe MVs of different lists in the co-located blocks of the co-locatedpictures according to the pseudocode below:

TABLE 1 Pseudocode for deriving temporal MV from the co-located blockfor TMVP When deriving the LX MV (X could be 0 or 1) of the temporalmotion candidate, the LY MV (Y could be 0 or 1) of the co-located blockis selected to derive the LX MV of the temporal motion candidate for thecurrent block. The selected LY MV of the co-located block is then scaledaccording to the POC distances as described in the following paragraph.If current picture has no backward prediction (which means there are noreference pictures have larger POC then current picture) LX MV of theco-located block is first selected. If the LX MV is not available, theL(1-X) is then selected. Otherwise (current picture has backwardprediction) LN MV of the co-located block is first selected. The N isset to the 1-co-located picture list (0 or 1). If the LN MV is notavailable, the L(1-N) is then selected.

The scaled motion vector 602 for temporal merge candidate is obtained asillustrated by the dotted line in FIG. 6A, which is scaled from theselected motion vector of the co-located block using the POC distance tb604 and POC distance td 606, where tb is defined to be the POCdifference between the reference picture of the current picture (e.g.,current reference 608) and the current picture (e.g., current picture610) and td is defined to be the POC difference between the referencepicture of the co-located picture (co-located reference 614) and theco-located picture (co-located picture 612). The reference picture indexof the temporal merge candidate is set equal to zero. A practicalrealization of the scaling process is described in the HEVCspecification. For a B-slice, two motion vectors, one is for referencepicture List 0 and the other is for reference picture List 1, areobtained and combined to make the bi-predictive merge candidate.

In the co-located block (e.g., co-located block 620) belonging to thereference frame, the position for the temporal candidate is selectedbetween candidates C₀ and C₁, as depicted in FIG. 6B. If block atposition C₀ is not available, is intra coded, or is outside of thecurrent CTU, position C₁ is used. Otherwise, position C₀ is used in thederivation of the temporal merge candidate.

Some coding standards (e.g., VVC Test Model 1) support sub-block-basedtemporal motion vector prediction (SbTMVP) method. Similar to thetemporal motion vector prediction (TMVP) in HEVC, SbTMVP uses the motionfield in the co-located picture to improve motion vector prediction andmerge mode for CUs in the current picture. The same co-located pictureused by TMVP is used for SbTMVP. SbTMVP differs from TMVP in thefollowing two main aspects:

-   -   1. TMVP predicts motion at CU level but SbTMVP predicts motion        at sub-CU level;    -   2. While TMVP selects the temporal motion vectors from the        co-located block in the co-located picture (the co-located block        is the bottom-right or center block relative to the current CU),        SbTMVP applies a motion shift to the temporal motion information        selected from the co-located picture, where the motion shift is        obtained from the motion vector from one of the spatial        neighboring blocks of the current CU.

The SbTMVP process is illustrated in FIGS. 6C-6D. SbTMVP (SbTMVP 632 ofFIG. 6D) predicts the motion vectors of the sub-CUs (e.g., sub-CU 634)within the current CU (current CU 636 of FIG. 6D) in two steps. In thefirst step, the spatial neighbor A1 (e.g., spatial neighbor 638) in FIG.6C is examined. If A1 has a motion vector that uses the co-locatedpicture (e.g., co-located picture 612 of FIG. 6A) as its referencepicture, this motion vector is selected to be the motion shift to beapplied (e.g., motion shift 630 of FIG. 6D). If no such motion vector isidentified, then the motion shift is set to zero-value vector (0, 0).The first available motion vector among the List 0 and List 1 MVs ofblock A1 is set to be the motion shift. This way, in SbTMVP, thecorresponding block may be more accurately identified, compared withTMVP, wherein the corresponding block (sometimes called co-locatedblock) is always in a bottom-right or center position relative to thecurrent CU. The pseudocode for determining the motion shift is below.

Pseudocode for determining the motion shift for the SbTMVP in VVC boolterminate = false; motion shift = 0; for (currRefListId = 0;currRefListId < (CurrentSliceType == B_SLICE ? 2 : 1) && !terminate;currRefListId++) { currRefPicList = RefPicList(LDC ? (ColFromL0Flag ?currRefListId : 1 − currRefListId) : currRefListId); if ((interDirA1 &(1 << currRefPicList)) && getRefPic(currRefPicList,refIdxA1[currRefListId]) == ColPic) { motion shift =mvA1[currRefListId]; terminate = true; break; } }

The variables and functions used in the table above are illustrated asfollows.

-   -   CoFromL0Flag: the syntax to indicate whether the co-located        picture is from the List 0 reference picture list    -   LDC: to indicate whether all reference pictures have smaller POC        values than the current picture    -   CurrentSliceType: the type of current slice (picture)    -   count: the available number of already derived merging        candidates    -   interDirA1: the interDir (1:L0, 2:L1 or 3:Bi) of the Nth merging        candidate    -   refIdxAl[0]: the L0 motion information (e.g. MV, ref. index) of        the Nth merging candidate    -   refIdxAl[1]: the L1 motion information (e.g. MV, ref. index) of        the Nth merging candidate    -   getRefPic(M,I): a function for getting a reference picture from        the reference picture List M with a reference index equal to I.

In the second step, the motion shift identified in Step 1 is applied(i.e. added to the current block's coordinates) to obtain sub-CU-levelmotion information (motion vectors and reference indices) from theco-located picture as shown in FIG. 6D. The example in FIG. 6D assumesthe motion shift is set to block A1's motion. In actual implementation,the motion shift may be set to any of the blocks A1, A2, B1, or B2'smotion.

First, a representative sub-CU is selected and the motion information ofthe corresponding block of this representative sub-CU is used as defaultmotion information. In the existing scheme of SbTMVP, the sub-CU locatedat the bottom-right of the center position of current CU is selected asthe representative sub-CU. When no valid motion information could bederived as the default motion information from the corresponding blockof the representative sub-CU, the SbTMVP candidate is regarded as notavailable. When default motion information is available, it goes to thenext step to derive the motion information for each sub-CU within thecurrent CU. Whenever no motion information is available for thecorresponding block of any sub-CU, the default motion information willbe used as its derived temporal motion for that sub-CU.

Then, for each sub-CU, the motion information of its corresponding block(the smallest motion grid that covers the center sample) in theco-located picture is used to derive the motion information for thesub-CU. After the motion information of the co-located sub-CU isidentified, it is converted to the motion vectors and reference indicesof the current sub-CU in a similar way as the TMVP process of HEVC,where temporal motion scaling is applied to align the reference picturesof the temporal motion vectors to those of the current CU.

It is noted that, in the current design, only the motion field within aco-located CTU plus one column on the right side of the co-located CTUin the co-located picture could be used for SbTMVP and TMVP derivationfor each CU. As shown in FIG. 7, only the motion information within theco-located CTU plus one column of motion information on the right of theco-located CTU (the CTU2 is the co-located CTU of the current CU in thisexample) could be used for the temporal my derivation for SbTMVP andTMVP. Hereafter, for the convenience of illustration, we would call thisco-located CTU plus one column as “valid area” for SbTMVP/TMVPderivation. Under this context, whenever a corresponding N×N block inthe co-located picture of a sub-CU is located outside the valid area,the corresponding N×N block is replaced with an alternative one locatedwithin the co-located CTU. The position of the alternative N×N block isderived by clipping the original position of the corresponding N×N blockto be located within the valid area using the below equation. In thebelow equation (position clipping process for each sub-CU),CurPicWidthInSamplesY and CurPicHeightInSamplesY are the width andheight of the coded picture, CTUWidthInSamplesX and CTUWidthInSamplesYare the width and height of the CTU, xCtb and yCtb are the horizontaland vertical position of the top-left sample of the co-located CTU.xColCtrCb and yColCtrCb are the horizontal and vertical position of therepresentative sample of the sub-CU, MotionShiftX and MotionShiftY arethe x and y components of the motion shift, respectively. The functionClip3(x,y,z) and Min(x,y) are defined as below.

${C{lip}\; 3\left( {x,y,z} \right)} = \left\{ {{\begin{matrix}x & ; & {z < x} \\y & ; & {z > y} \\z & ; & {otherwise}\end{matrix}{{Min}\left( {x,y} \right)}} = \left\{ \begin{matrix}x & ; & {x<=y} \\y & ; & {x > y}\end{matrix} \right.} \right.$

The location ( xColCb, yColCb ) of the co-located block inside theco-located picture is derived as follows. xColCb = Clip3( xCtb, Min(CurPicWidthInSamplesY − 1, xCtb + CTUWidthInSamplesY + 3 ), xColCtrCb +MotionShiftX ) ) yColCb = Clip3( yCtb, Min( CurPicHeightInSamplesY − 1,yCtb + CTUHeightInSamplesY − 1 ), yColCtrCb + MotionShiftY )

In VVC, a combined sub-block based merge list which contains both SbTMVPcandidate and affine merge candidates is used for the signaling ofsub-block based merge mode. The SbTMVP mode is enabled/disabled by asequence parameter set (SPS) flag. If the SbTMVP mode is enabled, theSbTMVP predictor is added as the first entry of the list of sub-blockbased merge candidates, and followed by the affine merge candidates. Thesize of the sub-block based merge list is signaled in SPS and themaximum allowed size of the sub-block based merge list is 5 in VVC.

The sub-CU size used in SbTMVP is fixed to be 8×8, and as done foraffine merge mode, SbTMVP mode is only applicable to the CU with bothwidth and height are larger than or equal to 8. Moreover, in currentVVC, for temporal motion field storage used by TMVP and SbTMVP, motionfield compression is performed at 8×8 granularity in contrast to the16×16 granularity in HEVC.

In some embodiments, the motion shift is always derived from the List 0my of the neighboring block; if the List 0 my is not available, the List1 my of the neighboring block is then used to derive the motion shiftfor SbTMVP. The pseudocode is described below:

Pseudocode for determining the motion shift for SbTMVP bool terminate =false; for (currRefListId = 0; currRefListId < (CurrentSliceType ==B_SLICE ? 2 : 1) && !terminate; currRefListId++) {currRefPicList =currRefListId; if ((interDirA1 & (1 << currRefPicList)) &&getRefPic(currRefPicList, refIdxA1[currRefListId]) == ColPic) { motionshift = mvA1[currRefListId]; terminate = true; break; } }

In some embodiments, the motion shift is always derived from the List 1my of the neighboring block; if the List 1 my is not available, the List0 my of the neighboring block is then used to derive the motion shiftfor SbTMVP. The pseudocode is described below:

Pseudocode for determining the motion shift for SbTMVP bool terminate =false; for (currRefListId = 0; currRefListId < (CurrentSliceType ==B_SLICE ? 2 : 1) && !terminate; currRefListId++) {currRefPicList =1−currRefListId; if ((interDirA1 & (1 << currRefPicList)) &&getRefPic(currRefPicList, refIdxA1[currRefListId]) == ColPic) { motionshift = mvA1[currRefListId]; terminate = true; break; } }

In some embodiments, whenever there is any corresponding block of asub-CU located outside the valid area, the zero vector is used as themotion shift vector to derive the SbTMVP. By doing so, the correspondingblocks of all the sub-CUs of current CU are guaranteed to be locatedwithin the valid area. Therefore, no position clipping process isrequired for each sub-CU. There are many ways to determine whether thereis any corresponding block of a sub-CU in current CU is located outsidethe valid area. In one example, the corresponding block of the top-leftN×N sub-CU and the corresponding block of the bottom-right N×N sub-CUare checked to see whether the two corresponding blocks are within thevalid area. If either one is located outside the valid area, zero vectoris used as the motion shift vector; otherwise (both corresponding blocksare located within the valid area), the derived motion shift is used forSbTMVP.

In some embodiments, whenever there is any corresponding block of asub-CU located outside the valid area, the SbTMVP is regarded as notavailable for the current CU.

In some embodiments, whenever there is any corresponding block of asub-CU located outside the valid area, the motion shift is modified toguarantee that the corresponding blocks of all the sub-CUs are locatedwithin the valid area. Therefore, no position clipping process isrequired for each sub-CU.

In some embodiments, zero vector is always used the motion shift for theSbTMVP derivation.

In some embodiments, it is proposed to use the default MV derived fromthe representative sub-CU as the MV of the sub-CU having a correspondingblock located outside the valid area.

FIG. 7 illustrates a block diagram for determining the valid area forderiving the TMVP and SbTMVP for a coding block (e.g., current CU 702)in a current picture (e.g., current picture 704) in accordance with someimplementations of the present disclosure. The valid area is an area inthe co-located picture (e.g., co-located picture 704′) in which acorresponding CU (e.g., corresponding CU 702′) to a current CU (e.g.,current CU 702) is being searched for the TMVP or SbTMVP. In someimplementations, the valid area is determined by the CTU (e.g., CTU2′)plus one column (e.g., one column TMV buffer 706) for deriving the TMVPand SbTMVP. The valid area constraint is a design for memory usagereduction. By constraining the valid area as the co-located CTU plus onecolumn, only the motion information within the valid area needs to bestored in the internal memory (e.g. cache) to reduce the average cost(time or energy) of accessing the temporal motion data from the outsidememory. Currently, the maximum CTU size is 128×128 in VVC (the maximumCTU size may be determined in the later stage for VVC profiles), and theCTU size could be set as less than 128×128 (e.g. 64×64 or 32×32). In oneexample when the CTU size is set to 64×64, the valid area is constrainedas the co-located 64×64 block plus one column. Since the design of thetemporal MV buffer for the maximum CTU is already there, it may beunwise to use a valid area smaller than the size of maximum CTU from thecoding efficiency perspective. In some embodiments, the valid area isalways fixed as the allowable maximum CTU size plus one column no matterwhat CTU size is in use.

In some embodiments, the valid area is modified to be just theco-located CTU.

In some embodiments, the valid area is the co-located CTU plus onecolumn when the CTU size is equal to the maximum CTU size. When the CTUsize is smaller than the maximum CTU size, the valid area is modified tobe the co-located CTU plus one column on the right of the co-located CTUand one row below the co-located CTU.

FIGS. 8A-8B illustrate a flowchart illustrating an exemplary process 800by which a video coder implements the techniques of deriving sub-blocktemporal motion vector predictors in accordance with someimplementations of the present disclosure. Although process 800 can be adecoding or an encoding process, for convenience, process 800 will bedescribed as a decoding process, performed by a video decoder (e.g., thevideo decoder 30 of FIG. 3).

As the first step, the decoder determines a co-located picture of thecurrent coding unit (805) (e.g., receiving a first syntax element fromthe bitstream that indicates whether a co-located picture of the currentframe is from a first list or a second list; then receiving a secondsyntax element from the bitstream which indicates which frame of theselected list is used as the co-located frame). For example, refer toFIG. 6A, the current CU 601 in the current picture 610 corresponds to aco-located Cu 601′ in co-located picture 612.

Next, the decoder locates a spatial neighbor block of the current codingunit (810). For example, refer to FIG. 6D, the current coding unit(e.g., current CU 636) has spatial neighbor 638 (block A1). In someembodiments, the spatial neighbor block is a coding unit or a sub-block.

After locating the spatial neighbor block, the decoder then determines amotion shift vector for the current coding unit (815). The motion shiftvector indicates a shift in spatial position between the current codingunit (e.g., current CU 636 in FIG. 6D) in the current picture (e.g.,current picture 610 in FIG. 6D) and a corresponding co-located block(e.g., spatial neighbor 638′ (block A1′) in FIG. 6D) in the co-locatedpicture (e.g., co-located picture 612 in FIG. 6D).

To determine the motion shift vector, the decoder sequentially examineseach of the motion vectors included in the List 0 of the spatialneighbor block (820). In accordance with a determining that a respectivemotion vector in the List 0 uses the co-located picture as therespective motion vector's reference picture (825): the decoder sets therespective motion vector in the List 0 as the motion shift vector (830)(e.g., motion shift vector 630), and forgoes examining subsequent motionvectors in the List 0 and motion vectors in the List 1 of the spatialneighbor block (835). As a result, the search for motion vectorconcludes and the first matching motion vector in the List 0 will beused as the motion shift vector. In other words, the decoder alwaysfirst checks the motion vectors included in the List 0 of the spatialneighbor block before checking the List 1.

On the other hand, in accordance with a determination that no respectivemotion vector in the List 0 uses the co-located picture as the referencepicture (840), the decoder sequentially examines each of the motionvectors included in the List 1 of the spatial neighbor block (845). Thatis to say, the decoder only checks the List 1 of the spatial neighborblock of motion vectors if and only if the search of motion vectors inthe List 0 returns negative results.

While searching for motion vectors in the List 1 of the spatial neighborblock, in accordance with a determination that a respective motionvector in the List 1 uses the co-located picture as the respectivemotion vector's reference picture (850): the decoder sets the respectivemotion vector in the List 1 as the motion shift vector (855), andforgoes examining subsequent motion vectors in the List 1 (860). That isto say, the first matching motion vector in the List 1 will be used asthe motion shift vector. In accordance with a determination that norespective motion vector in the List 1 uses the co-located picture asthe respective motion vector's reference picture (865), the decoder setsthe motion shift vector to be a zero-value vector (870). As a result,the corresponding coding unit and the current coding unit are in thesame relative position with respect to the co-located picture and thecurrent picture (e.g., no shift in motion between the current codingunit and the corresponding coding unit).

Finally, the decoder reconstructs a sub-block-based temporal motionvector for a respective sub-block of a plurality of sub-blocks in thecurrent coding unit from a corresponding sub-block in the co-locatedpicture based on the motion shift vector (875). For example, refer toFIG. 6D, sub-block temporal motion vector predictor 632 is constructedby using the motion shift vector 630 to locate the correspondingsub-block temporal motion vector 631 after scaling (e.g., the scalingprocess described with respect to FIG. 6A and the related description).In some embodiments, a sub-block includes one or two temporal motionvectors, from the List 0 and the List 1.

In some embodiments, the reconstructing the sub-block-based temporalmotion vector for the respective sub-block of the plurality ofsub-blocks in the current coding unit from the corresponding sub-blockin the co-located picture based on the motion shift vector includespredicting sub-block-based temporal motion vectors for a respectivesub-block of a plurality of sub-blocks in the current coding unit,including: searching, within a predefined area (e.g., valid area) in theco-located picture, a co-located sub-block corresponding to therespective sub-block based on the motion shift vector; in accordancewith a determination that the co-located sub-block exists within thepredefined area in the co-located picture: identifying one or two motionvectors of the co-located sub-block; and setting the sub-block-basedtemporal motion vectors for the respective sub-block as the one or twomotion vectors scaled based on a first picture order count (POC)distance (e.g., POC distance tb in FIG. 6A) between the current pictureand a reference picture of the current picture, and a second POCdistance (e.g., POD distance td in FIG. 6A) between the co-locatedpicture and a reference picture of the co-located picture. In someembodiments, in accordance with a determination that the co-locatedsub-block does not exist within the predefined area in the co-locatedpicture, the sub-block-based temporal motion vectors for thecorresponding sub-block are set to be zero-value motion vectors. In someother embodiments, in accordance with a determination that theco-located sub-block does not exist within the predefined area in theco-located picture, an alternative sub-block within the predefined areain the collocated picture is set as the corresponding sub-block. Forexample, the alternative sub-block is the boundary sub-block within thepredefined area that is closest to the co-located sub-block.

In some embodiments, the predefined area has a size equal to the maximumallowable CTU size plus one column, regardless of the size of the CTUincluding the co-located coding unit.

In some embodiments, the decoder checks the motion vectors in the List 1of the spatial neighbor block first before checking the List 0.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the implementationsdescribed in the present application. A computer program product mayinclude a computer-readable medium.

The terminology used in the description of the implementations herein isfor the purpose of describing particular implementations only and is notintended to limit the scope of claims. As used in the description of theimplementations and the appended claims, the singular forms “a,” “an,”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will also be understood that theterm “and/or” as used herein refers to and encompasses any and allpossible combinations of one or more of the associated listed items. Itwill be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, elements, and/or components, but do not preclude thepresence or addition of one or more other features, elements,components, and/or groups thereof.

It will also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first electrode could be termeda second electrode, and, similarly, a second electrode could be termed afirst electrode, without departing from the scope of theimplementations. The first electrode and the second electrode are bothelectrodes, but they are not the same electrode.

The description of the present application has been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications, variations, and alternative implementations will beapparent to those of ordinary skill in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. The embodiment was chosen and described in order to bestexplain the principles of the invention, the practical application, andto enable others skilled in the art to understand the invention forvarious implementations and to best utilize the underlying principlesand various implementations with various modifications as are suited tothe particular use contemplated. Therefore, it is to be understood thatthe scope of claims is not to be limited to the specific examples of theimplementations disclosed and that modifications and otherimplementations are intended to be included within the scope of theappended claims.

According to the present disclosure, a method of decoding a currentcoding unit in a current picture comprises: determining a co-locatedpicture for the current picture; determining a motion shift vector forthe current coding unit according to a motion vector of a spatialneighbor block of the current coding unit, wherein the motion shiftvector indicates a shift in spatial position between a respectivesub-block of a plurality of sub-blocks in the current coding unit in thecurrent picture and a corresponding sub-block in the co-located picture;and reconstructing a sub-block-based temporal motion vector for therespective sub-block of the plurality of sub-blocks in the currentcoding unit from the corresponding sub-block in the co-located picturebased on the motion shift vector.

According to an embodiment of the present disclosure, the determiningthe motion shift vector for the current coding unit according to themotion vector of the spatial neighbor block of the current coding unitcomprises: in accordance with a determination that a motion vectorrelated to a first reference picture list for the spatial neighbor blockuses the co-located picture as a reference picture for the motion vectorrelated to the first reference picture list, setting the motion vectorrelated to the first reference picture list as the motion shift vector;or in accordance with a determination that a motion vector related tothe first reference picture list does not use the co-located picture asa reference picture for the motion vector related to the first referencepicture list: in accordance with a determination that a motion vectorrelated to a second reference picture list for the spatial neighborblock uses the co-located picture as a reference picture for the motionvector related to the second reference picture list, setting the motionvector related to the second reference picture list as the motion shiftvector; or in accordance with a determination that a motion vectorrelated to the second reference picture list for the spatial neighborblock does not use the co-located picture as a reference picture for themotion vector related to the second reference picture list, setting themotion shift vector to be a zero-value vector.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining, within a predefined area in theco-located picture, a co-located sub-block corresponding to therespective sub-block based on the motion shift vector; and setting thesub-block-based temporal motion vector for the respective sub-block asone or two scaled motion vectors derived based on one or two motionvectors of the co-located sub-block, a first picture order count (POC)distance between the current picture and a reference picture of thecurrent picture, and a second POC distance between the co-locatedpicture and a reference picture of the co-located picture.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining whether the co-located sub-block iswithin a predefined area in the co-located picture.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: in accordance with a determination that theco-located sub-block does not exist within the predefined area in theco-located picture: setting the sub-block-based temporal motion vectorsfor the corresponding sub-block to be zero-value motion vectors.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: in accordance with a determination that theco-located sub-block does not exist within the predefined area in theco-located picture: setting an alternative sub-block within thepredefined area in the collocated picture as the correspondingsub-block, wherein the alternative sub-block is the boundary sub-blockwithin the predefined area that is closest to the co-located sub-block.

According to an embodiment of the present disclosure, the spatialneighbor block of the current coding unit is a coding unit or asub-block of a coding unit.

According to an embodiment of the present disclosure, the predefinedarea has a size equal to maximum allowable CTU size plus one column,regardless of a size of an CTU including the co-located sub-block.

According to an embodiment of the present disclosure, the maximumallowable CTU size is 128×128.

According to the present disclosure, a computing device comprises: oneor more processors; memory coupled to the one or more processors; and aplurality of programs stored in the memory that, when executed by theone or more processors, cause the computing device to perform operationscomprising: determining a co-located picture for the current picture;determining a motion shift vector for the current coding unit accordingto a motion vector of a spatial neighbor block of the current codingunit, wherein the motion shift vector indicates a shift in spatialposition between a respective sub-block of a plurality of sub-blocks inthe current coding unit in the current picture and a correspondingsub-block in the co-located picture; and reconstructing asub-block-based temporal motion vector for the respective sub-block ofthe plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector.

According to an embodiment of the present disclosure, the determiningthe motion shift vector for the current coding unit according to themotion vector of the spatial neighbor block of the current coding unitcomprises: in accordance with a determination that a motion vectorrelated to a first reference picture list for the spatial neighbor blockuses the co-located picture as a reference picture for the motion vectorrelated to the first reference picture list, setting the motion vectorrelated to the first reference picture list as the motion shift vector;or in accordance with a determination that a motion vector related tothe first reference picture list does not use the co-located picture asa reference picture for the motion vector related to the first referencepicture list: in accordance with a determination that a motion vectorrelated to a second reference picture list for the spatial neighborblock uses the co-located picture as a reference picture for the motionvector related to the second reference picture list, setting the motionvector related to the second reference picture list as the motion shiftvector; or in accordance with a determination that a motion vectorrelated to the second reference picture list for the spatial neighborblock does not use the co-located picture as a reference picture for themotion vector related to the second reference picture list, setting themotion shift vector to be a zero-value vector.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining, within a predefined area in theco-located picture, a co-located sub-block corresponding to therespective sub-block based on the motion shift vector; and setting thesub-block-based temporal motion vector for the respective sub-block asone or two scaled motion vectors derived based on one or two motionvectors of the co-located sub-block, a first picture order count (POC)distance between the current picture and a reference picture of thecurrent picture, and a second POC distance between the co-locatedpicture and a reference picture of the co-located picture.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining whether the co-located sub-block iswithin a predefined area in the co-located picture.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: in accordance with a determination that theco-located sub-block does not exist within the predefined area in theco-located picture: setting the sub-block-based temporal motion vectorsfor the corresponding sub-block to be zero-value motion vectors.

According to an embodiment of the present disclosure, the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: in accordance with a determination that theco-located sub-block does not exist within the predefined area in theco-located picture: setting an alternative sub-block within thepredefined area in the collocated picture as the correspondingsub-block, wherein the alternative sub-block is the boundary sub-blockwithin the predefined area that is closest to the co-located sub-block.

According to an embodiment of the present disclosure, the spatialneighbor block of the current coding unit is a coding unit or asub-block of a coding unit.

According to an embodiment of the present disclosure, the predefinedarea has a size equal to maximum allowable CTU size plus one column,regardless of a size of an CTU including the co-located sub-block.

According to an embodiment of the present disclosure, the maximumallowable CTU size is 128×128.

According to the present disclosure, a non-transitory computer readablestorage medium stores a plurality of programs for execution by acomputing device having one or more processors. And the plurality ofprograms, when executed by the one or more processors, cause thecomputing device to perform operations comprising: determining aco-located picture for the current picture; determining a motion shiftvector for the current coding unit according to a motion vector of aspatial neighbor block of the current coding unit, wherein the motionshift vector indicates a shift in spatial position between a respectivesub-block of a plurality of sub-blocks in the current coding unit in thecurrent picture and a corresponding sub-block in the co-located picture;and reconstructing a sub-block-based temporal motion vector for therespective sub-block of the plurality of sub-blocks in the currentcoding unit from the corresponding sub-block in the co-located picturebased on the motion shift vector.

According to an embodiment of the present disclosure, the determiningthe motion shift vector for the current coding unit according to themotion vector of the spatial neighbor block of the current coding unitcomprises: in accordance with a determination that a motion vectorrelated to a first reference picture list for the spatial neighbor blockuses the co-located picture as a reference picture for the motion vectorrelated to the first reference picture list, setting the motion vectorrelated to the first reference picture list as the motion shift vector;or in accordance with a determination that a motion vector related tothe first reference picture list does not use the co-located picture asa reference picture for the motion vector related to the first referencepicture list: in accordance with a determination that a motion vectorrelated to a second reference picture list for the spatial neighborblock uses the co-located picture as a reference picture for the motionvector related to the second reference picture list, setting the motionvector related to the second reference picture list as the motion shiftvector; or in accordance with a determination that a motion vectorrelated to the second reference picture list for the spatial neighborblock does not use the co-located picture as a reference picture for themotion vector related to the second reference picture list, setting themotion shift vector to be a zero-value vector.

What is claimed is:
 1. A method of decoding a current coding unit in acurrent picture, the method comprising: determining a co-located picturefor the current picture; determining a motion shift vector for thecurrent coding unit according to a motion vector of a spatial neighborblock of the current coding unit, wherein the motion shift vectorindicates a shift in spatial position between a respective sub-block ofa plurality of sub-blocks in the current coding unit in the currentpicture and a corresponding sub-block in the co-located picture; andreconstructing a sub-block-based temporal motion vector for therespective sub-block of the plurality of sub-blocks in the currentcoding unit from the corresponding sub-block in the co-located picturebased on the motion shift vector.
 2. The method according to claim 1,wherein determining the motion shift vector for the current coding unitaccording to the motion vector of the spatial neighbor block of thecurrent coding unit comprises: in accordance with a determination that amotion vector related to a first reference picture list for the spatialneighbor block uses the co-located picture as a reference picture forthe motion vector related to the first reference picture list, settingthe motion vector related to the first reference picture list as themotion shift vector; or in accordance with a determination that a motionvector related to the first reference picture list does not use theco-located picture as a reference picture for the motion vector relatedto the first reference picture list: in accordance with a determinationthat a motion vector related to a second reference picture list for thespatial neighbor block uses the co-located picture as a referencepicture for the motion vector related to the second reference picturelist, setting the motion vector related to the second reference picturelist as the motion shift vector; or in accordance with a determinationthat a motion vector related to the second reference picture list forthe spatial neighbor block does not use the co-located picture as areference picture for the motion vector related to the second referencepicture list, setting the motion shift vector to be a zero-value vector.3. The method of claim 1, wherein the reconstructing the sub-block-basedtemporal motion vector for the respective sub-block of the plurality ofsub-blocks in the current coding unit from the corresponding sub-blockin the co-located picture based on the motion shift vector comprises:determining, within a predefined area in the co-located picture, aco-located sub-block corresponding to the respective sub-block based onthe motion shift vector; and setting the sub-block-based temporal motionvector for the respective sub-block as one or two scaled motion vectorsderived based on one or two motion vectors of the co-located sub-block,a first picture order count (POC) distance between the current pictureand a reference picture of the current picture, and a second POCdistance between the co-located picture and a reference picture of theco-located picture.
 4. The method of claim 1, wherein the reconstructingthe sub-block-based temporal motion vector for the respective sub-blockof the plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining whether the co-located sub-block iswithin a predefined area in the co-located picture.
 5. The method ofclaim 4, wherein the reconstructing the sub-block-based temporal motionvector for the respective sub-block of the plurality of sub-blocks inthe current coding unit from the corresponding sub-block in theco-located picture based on the motion shift vector comprises: inaccordance with a determination that the co-located sub-block does notexist within the predefined area in the co-located picture: setting thesub-block-based temporal motion vectors for the corresponding sub-blockto be zero-value motion vectors.
 6. The method of claim 4, wherein thereconstructing the sub-block-based temporal motion vector for therespective sub-block of the plurality of sub-blocks in the currentcoding unit from the corresponding sub-block in the co-located picturebased on the motion shift vector comprises: in accordance with adetermination that the co-located sub-block does not exist within thepredefined area in the co-located picture: setting an alternativesub-block within the predefined area in the collocated picture as thecorresponding sub-block, wherein the alternative sub-block is theboundary sub-block within the predefined area that is closest to theco-located sub-block.
 7. The method of claim 1, wherein the spatialneighbor block of the current coding unit is a coding unit or asub-block of a coding unit.
 8. The method of claim 3, wherein thepredefined area has a size equal to maximum allowable CTU size plus onecolumn, regardless of a size of an CTU including the co-locatedsub-block.
 9. The method of claim 8, wherein the maximum allowable CTUsize is 128×128.
 10. A computing device comprising: one or moreprocessors; memory coupled to the one or more processors; and aplurality of programs stored in the memory that, when executed by theone or more processors, cause the computing device to perform operationscomprising: determining a co-located picture for the current picture;determining a motion shift vector for the current coding unit accordingto a motion vector of a spatial neighbor block of the current codingunit, wherein the motion shift vector indicates a shift in spatialposition between a respective sub-block of a plurality of sub-blocks inthe current coding unit in the current picture and a correspondingsub-block in the co-located picture; and reconstructing asub-block-based temporal motion vector for the respective sub-block ofthe plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector.
 11. The computing device according to claim 10, whereinthe determining the motion shift vector for the current coding unitaccording to the motion vector of the spatial neighbor block of thecurrent coding unit comprises: in accordance with a determination that amotion vector related to a first reference picture list for the spatialneighbor block uses the co-located picture as a reference picture forthe motion vector related to the first reference picture list, settingthe motion vector related to the first reference picture list as themotion shift vector; or in accordance with a determination that a motionvector related to the first reference picture list does not use theco-located picture as a reference picture for the motion vector relatedto the first reference picture list: in accordance with a determinationthat a motion vector related to a second reference picture list for thespatial neighbor block uses the co-located picture as a referencepicture for the motion vector related to the second reference picturelist, setting the motion vector related to the second reference picturelist as the motion shift vector; or in accordance with a determinationthat a motion vector related to the second reference picture list forthe spatial neighbor block does not use the co-located picture as areference picture for the motion vector related to the second referencepicture list, setting the motion shift vector to be a zero-value vector.12. The computing device of claim 10, wherein the reconstructing thesub-block-based temporal motion vector for the respective sub-block ofthe plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining, within a predefined area in theco-located picture, a co-located sub-block corresponding to therespective sub-block based on the motion shift vector; and setting thesub-block-based temporal motion vector for the respective sub-block asone or two scaled motion vectors derived based on one or two motionvectors of the co-located sub-block, a first picture order count (POC)distance between the current picture and a reference picture of thecurrent picture, and a second POC distance between the co-locatedpicture and a reference picture of the co-located picture.
 13. Thecomputing device of claim 10, wherein the reconstructing thesub-block-based temporal motion vector for the respective sub-block ofthe plurality of sub-blocks in the current coding unit from thecorresponding sub-block in the co-located picture based on the motionshift vector comprises: determining whether the co-located sub-block iswithin a predefined area in the co-located picture.
 14. The computingdevice of claim 13, wherein the reconstructing the sub-block-basedtemporal motion vector for the respective sub-block of the plurality ofsub-blocks in the current coding unit from the corresponding sub-blockin the co-located picture based on the motion shift vector comprises: inaccordance with a determination that the co-located sub-block does notexist within the predefined area in the co-located picture: setting thesub-block-based temporal motion vectors for the corresponding sub-blockto be zero-value motion vectors.
 15. The computing device of claim 13,wherein the reconstructing the sub-block-based temporal motion vectorfor the respective sub-block of the plurality of sub-blocks in thecurrent coding unit from the corresponding sub-block in the co-locatedpicture based on the motion shift vector comprises: in accordance with adetermination that the co-located sub-block does not exist within thepredefined area in the co-located picture: setting an alternativesub-block within the predefined area in the collocated picture as thecorresponding sub-block, wherein the alternative sub-block is theboundary sub-block within the predefined area that is closest to theco-located sub-block.
 16. The computing device of claim 10, wherein thespatial neighbor block of the current coding unit is a coding unit or asub-block of a coding unit.
 17. The computing device of claim 12,wherein the predefined area has a size equal to maximum allowable CTUsize plus one column, regardless of a size of an CTU including theco-located sub-block.
 18. The computing device of claim 17, wherein themaximum allowable CTU size is 128×128.
 19. A non-transitory computerreadable storage medium storing a plurality of programs for execution bya computing device having one or more processors, wherein the pluralityof programs, when executed by the one or more processors, cause thecomputing device to perform operations comprising: determining aco-located picture for the current picture; determining a motion shiftvector for the current coding unit according to a motion vector of aspatial neighbor block of the current coding unit, wherein the motionshift vector indicates a shift in spatial position between a respectivesub-block of a plurality of sub-blocks in the current coding unit in thecurrent picture and a corresponding sub-block in the co-located picture;and reconstructing a sub-block-based temporal motion vector for therespective sub-block of the plurality of sub-blocks in the currentcoding unit from the corresponding sub-block in the co-located picturebased on the motion shift vector.
 20. The non-transitory computerreadable storage medium according to claim 19, wherein the determiningthe motion shift vector for the current coding unit according to themotion vector of the spatial neighbor block of the current coding unitcomprises: in accordance with a determination that a motion vectorrelated to a first reference picture list for the spatial neighbor blockuses the co-located picture as a reference picture for the motion vectorrelated to the first reference picture list, setting the motion vectorrelated to the first reference picture list as the motion shift vector;or in accordance with a determination that a motion vector related tothe first reference picture list does not use the co-located picture asa reference picture for the motion vector related to the first referencepicture list: in accordance with a determination that a motion vectorrelated to a second reference picture list for the spatial neighborblock uses the co-located picture as a reference picture for the motionvector related to the second reference picture list, setting the motionvector related to the second reference picture list as the motion shiftvector; or in accordance with a determination that a motion vectorrelated to the second reference picture list for the spatial neighborblock does not use the co-located picture as a reference picture for themotion vector related to the second reference picture list, setting themotion shift vector to be a zero-value vector.